Shielding for microwave lens apparatus



June 11, 1957 c. F. CRAWFORD Erm. 2,795,784

SHIELDING FOR MICROWAVE LENS APPARATUS Filed Feb.l 16, 1953 ZLH .75 Uff TTORNEY SHIELDING Fon MIcRowAvE LENS APPARATUS Carl Francis Crawford, Pennsanken, N. J., John Randolph Ford, N arberth, Pa., and Edwin Smith Lewis, Merchantville, N. 3., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application February 16, 1953, Serial No. 337,024

2 Claims. (Cl. 343-841) The present invention is related to shielding for microwave apparatus, and particularly for shielding the eld between a microwave lens and its near focal point from extraneous iniluence.

In microwave apparatus employing a lens, either for focusing radiated or received microwaves, the associated equipment includes a transmitter or receiver, or both, and often other equipment as well. The equipment is usually enclosed in metallic containers for excluding stray fields and also for preventing the escape of stray ields. It is often desirable to group the containers, and the equipment therein, as closely as possible together, and in close and convenient order about the antenna element, such as a waveguide, dipole, or the like, which is placed at or near the close focal point of the lens to receive or transmit energy focused by the lens.

The containers should not be interposed between the antenna element and the active focusing portion of the lens because such interposition would obstruct trans mission or reception. However, the equipment containers may be placed immediately adjacent the volume between the antenna element and the active lens portion. Un fortunately, due to diffraction and reilection of the iields between the lens and the antenna element, the placing of these containers adjacent this volume also interferes with the efficient transmission or reception of energy. The fields within the volume between the antenna element and the active lens portion thus are distorted from their desired values, and the distribution of the energy across the lens for transmission from, or to the element for radiation or reception are changed from what their values would be without the adjacent containers. A simple metaliic shield about the critical lens volume causes the same diiiculty to persist, or even to be made greater, because of glancing reilections from the shield. Freedom of placement of the containers is therefore not available, as they cannot be piaced closely adjacent the volume described. lf so placed, the energy pattern of the lens and antenna element is disturbed and distorted from the expected pattern. it will be apparent also that one may desire to place other objects of a metallic nature closely adjacent to the volume described, or to, move other equipment about in the near vicinity of this volume, and for the same reasons, such placement or movement has not heretofore been possible, without disturbing the electromagnetic energy pattern of the lens and antenna element.

it is an object of the present invention to substantially eliminate the influence of objects closely adjacent a cooperating antenna elernent and lens upon the radiation pattern of the lens and element.

lt is another object of the invention to substantially eliminate restrictions on the placement of associated apparatus closely adjacent the volume between the antenna element and the active lens portion, or lens, of an antenna element and lens combination.

A further object of the invention is to assure greater freedom of placement or movement of equipment closely adjacent the volume described.

Another object of the invention is to protect the volume 2,795,784 Patented June ll, i957 between an antenna element and a cooperating lens from extraneous influences arising from closely adjacent discontinuities and which may deleteriously affect the energy pattern.

In accordance with the invention, a metallic shield encircles the volume between the antenna element and the cooperating lens, the shield having an internal projection also encircling the volume, preferably in a plane normal to the lens axis. This internal projection is coated preferably on all the exposed surfaces, including the edges, with a highly resistive, energy absorbent material, for example, colloidal graphite, carbon, or the like. Further, there are preferably a plurality of such coated encircling projections which have their internal edges on the surface of the volume. For convenience and deiniteness, this volume may be y.defined by straight lines from the virtual source provided by the antenna `element (which virtual source may be substantially aline or a point) `to the outermost bundary of the active lensfsurface.

The foregoing and other objects, advantages, and novel features of the invention will be more fully apparent from the following description when read in connection with the accompanying drawing, the sole figure of which is a schematic View, partially in cross-section, of a preferred embodiment of the invention.

Referring to the drawing; there is illustrated an arrangement of metallic containers 10, l2, 14, 16 and 18 which house component parts of a microwave system. The system may include an antenna including a lens 32, illustrated as dielectric, although other types of microwave lenses may be employed. The lens includes also an antenna element, typiied by an electromagnetic horn 34. The horn 34 is connected by a waveguide 36 to the component l2. The horn 34, considering now the transmitting function of the antenna, in radiating energy, affords a virtual point source for the radiation at point 33, preferably the near focal point of lens 32, or at least very close to thatpoint on the optical axis ofy lens 32. For convenience, the point 38 Iwill be considered'as coincident with the lens 32 focal point, although it may in fact be displaced. The View of the Ihorn and lens may be considered as taken in a plane through the point 38 and a diameter of the lens. If the lens is cylindrical, the view may be considered as taken in a plane normal to the line dened by intersection of the axial plane and a lens surface.

A metallic shield 40, frustro-conical for a spherical lens, completely encircles the volume demarcated by straight lines such as 42 and 44 between point 38 and the lens edges. As would normally be the case, the entire lens is considered active, that is, to its edges the lens focuses energy from the horn, to make available the complete lens aperture. The metallic shield 40 is interposed between the volume, conical if the lens is spherical and wedge shaped if the lens is cylindrical, demarcated by the straight lines from point 38 such as lines 42 and 44 and any of the component containers. Further, the shield 4G is spaced, preferably of the order of two wavelengths from the conical volume. On its inside the shield i0 has a plurality of projections 46, 48, Si) and 52 which also encircle the described volume, with their inner edges preferably in contact with the outer contour of the volume described, and each projection preferably lying substantially in a plane normal to the optical axis. The projections are preferably coated with a highly resistive or energy absorbent coating at least on the side facing the incident radiation. Such coatings are Well known in the microwave art, and may be, for example, carbon or colloidal graphite. Thus, it the antenna is used for radiation, the coating is at least on the side of the projection facing the point 33, whereas if used for reception, the coating -is at least on the lside of the projection facing the lens 32. For reasons pointed out hereinafter, the coating `should be on both faces of each projection, and it is also well to have the coating on the inner projection edges.

The shield 40 need not extend to the lens 32, but sufficient shield is afforded to prevent any of the component containers or other metallic objects near the volume described from receiving or reflecting radiation from or to this volume` The shield may join the waveguide 34 for best results. If, however, no waveguide, but some alternative is employed, the'shiel-d should exclude by inter position, radiation from the entire volume described from anywhere the disturbing objects may be placed, in order to prevent undesiredv antenna energy pattern deterioration.

Preferably, the shield 40 termination in the dimension.

parallel to the optical axis and nearer the said lens lies in a plane normal to the opticalaxis of the lens, and at this termination is one of the projections lying substantially in the plane of the termination. At .its other termination, the shield preferably is continuously joined to the horn 34. When referring to a plane normal to the optical axis, rit will be understood'that for cylindrical lenses, an analogous plane is intended, or an axis of the `lens lying in'the axial plane. Such an axis may be the central line defined by the intersection of the axial plane and a second plane norf mal to the axial plane and normal to the straight line inter-` section of the axial plane and cylindrical lens surface.

in operation, the metallic shield preventsA stray radiation from entering the volume described, and'al'so prevents radiation from the volume from being reflected by `the containers and other metallicvobjects to the detriment of the antenna pattern. However, the shield alone, because l of glancng'reections of energy from the virtual source 38 in transmission, or to the source 38 in reception, adversely affects the antenna energy pattern. With the projections provided, however, itis apparent that these projections may be placed to prevent glancingly reflected energy from the shield from re-entering a path 'within' the volume which path might carry such energy to the lens or to the source. The projections are placed and made of a depth which first prevents any primary energy from near point 38, such yas from a waveguide 34 wall; or from the desired active portion of the lens from striking the internal surface of the shield 40. so is intercepted by the projection, av large portion being absorbed by the coating, and the remainder vbeing reected. Again, the projections are made sufficiently deep so that such reflection is toward an adjacent projection, which the energy reaches after a glancing reflection on the shield 40 and the energy is again absorbed in part and reected in part by the adjacent projection coating. The path of the remaining energy, after this second part absorption, at least in most cases, will be diametrically across the shield to be absorbed by further reflections on I the energy absorbently coated surfaces, before or after achieved without further treatment, the internal surface o of shield 40 proper is preferably not so coated. It is well to coat exposed edges of the projections as well to reduce refractive effects at such edges.

The invention thus preventsv stray reections and refractions from adversely affecting the energy pattern of the antenna.' It -is apparent that the lens 32 may be dielectric, as shown, or of the metal .lens type (not shown). Although the invention has been described as though it pertained to a radar (radio echo detection and ranging system), it is apparent that it may also be -used to advantage in other microwave systems. For example, the invention might be employed in a microwave relay link. In such. `a system two antennas'facing dilferent directions are often employed. YThe components may be interconnected by connections 28 and 3l), and a second waveguide 36 may lead to the second antenna. The other components (not shown) might be arranged about the second antenna in a mirror image arrangement of the first antenna and the coinponents shown, with a second shield having internal projections similar to the first.

Thus the invention includes a novel means of preventing deterioration of an antenna energy pattern which might arise due to reflection or refraction from closely positioned objects. yThe means includes a metallic shield encircling the volume between the active lens portion and the near point from or to which the Ilens is to direct radiation which is substantially the near focal point, and encircling projections from the inner shield having an energy absorbent coating. Y

What is claimed: l. In a microwave system, the combination comprising an antenna having a focusing lens and an antenna ele- .v optical axis.

Instead, energy directed to do References Cited in the file of this patent UNITED STATES PATENTS 2,162,324 Worrall lune 13, 1939 2,273,443 Ohl Feb. 17, 1942 2,283,568 Ohl May 19, 1942 2,317,464 Katzin Apr, 27, 1943 2,464,006 Tiley Mar. 8, 1949 2,599,944 Salisbury lune 10, 1952 2,636,125 Southworth Apr. 21, 1953 2,717,312 Taylor Sept. 6, 1955 OTHER REFERENCES Propagation of Short Radio Waves, vol. 13, M. I. T. Radiation Laboratory Series, McGraw-Hill Book Co., Inc., New York, 1951, pages 3 and 4. (Copy in Div. 44.)

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